VUV Ionization Induced Reaction of Neutral Au2Al2O3 Clusters With

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VUV Ionization Induced Reaction of Neutral AuAlO Clusters With Methane Jiao-Jiao Chen, Yuan Yang, Yan-Xia Zhao, and Sheng-Gui He J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b00157 • Publication Date (Web): 27 Feb 2018 Downloaded from http://pubs.acs.org on March 4, 2018

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The Journal of Physical Chemistry

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VUV Ionization Induced Reaction of Neutral

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Au2Al2O3 Clusters with Methane

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Jiao-Jiao Chen†,‡,§, Yuan Yang†,‡,§, Yan-Xia Zhao*,†,‡, Sheng-Gui He*,†,‡,§

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†

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Chemistry, Chinese Academy of Sciences, Beijing 100190, China

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‡

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Excellence in Molecular Sciences, Beijing 100190, China

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§

State Key Laboratory for Structural Chemistry of Unstable and Stable Species, Institute of

Beijing National Laboratory for Molecular Sciences, CAS Research/Education Center of

University of Chinese Academy of Sciences, Beijing 100049, China

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*E-mail: [email protected] (Y.-X. Z.); [email protected] (S.-G. H.); phone: +86-10-

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62568330; fax: +86-10-62559373.

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ACS Paragon Plus Environment

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The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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ABSTRACT

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Identification of molecular-level mechanisms of methane activation is very important in order to

3

transform the most stable alkane molecule into value-added chemicals. Herein, by employing a

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home-made time-of-flight mass spectrometer coupled with a vacuum ultraviolet (VUV) laser

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system, the reactions of neutral heteronuclear metal oxide clusters AuxAlyOz (x = 1, 2) with CH4

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have been investigated. The interesting channel of Au2Al2O3H+ production was observed for the

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interaction of Au2Al2O3 cluster with CH4 after VUV ionization, suggesting that the methyl

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radical was generated. Combined with the theoretical calculations, this study proposes that VUV

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ionization of the neutral adsorption complex (Au2Al2O3CH4) creates active cation

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(Au2Al2O3CH4+)* to induce C–H bond activation and the methyl radical formation. The

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photochemistry of this cluster study provides molecular-level insights into the interactions

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between methane and active sites in related photocatalytic processes.

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1. INTRODUCTION

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Methane, the main constituent of natural gas and biogas, is widely employed as a raw material

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for the production of value-added chemicals.1-3 The efficient activation and direct conversion of

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methane remain an extreme challenge due to the high stability of the C–H bonds4,5 and most of

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the catalytic processes are typically performed under harsh reaction conditions.6-8 The

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photocatalysis technology considered as a mild and sustainable strategy has been extensively

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applied in methane conversion to produce ethane9,10, methanol11-13, benzene,14 and so on.15,16 It

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has been proposed that these photocatalytic methane activation and conversion are driven by

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separated electrons and holes upon photoexcitation of electron-hole pairs in the photocatalysts.17

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Due to the complexity of condensed-phase systems, the detailed mechanisms of the interactions

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between methane and photoactive sites at the molecular level are still elusive.

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Gas phase studies on isolated atomic clusters have been generally considered as an ideal arena

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for probing reaction mechanisms relevant with condensed-phase systems under controlled and

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reproducible conditions.18-28 A lot of works have identified the C–H bond activation and methane

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transformation facilitated by cationic metal oxide clusters such as V4O10+,29 OsO4+,30 AlVO4+,31

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AuNbO3+,32 and so on.33,34 The cationic clusters can be considered as models to mimic hole-

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mediated processes after photoexcitation on the surface of photocatalysts. The active hole center

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may be generated by exciting one electron from neutral species.35 Therefore, studying the

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photoreactions of neutral clusters with methane is important to understand the relevant reaction

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mechanisms. Photo-induced methane activation mediated with neutral metal atoms like Mn, Zn,

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and Ti,36,37 or diatomic metal oxides such as NbO, TaO, and TiO38-40 has been reported. These

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studies were focused on methane activation by oxidative addition mechanism, and the role of

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light irradiation was to provide excess energy to overcome the activation barrier.


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In this work, by employing a home-made time-of-flight mass spectrometer (TOF-MS) coupled

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with a vacuum ultraviolet (VUV) laser system,41 we investigated the reactions of neutral

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heteronuclear metal oxide clusters AuxAlyOz (x = 1, 2) with CH4. The methyl radical elimination

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was found only for the interaction of CH4 with Au2Al2O3 cluster after VUV ionization. In

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conjunction with density functional calculations, this study proposes that the VUV ionization

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prepares the cationic species which mimics the active hole center for methane activation and

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conversion. Such photo-induced reaction may serve as a model to understand the interactions of

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methane with photoactive sites in related condensed-phase systems. It is noteworthy that the

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reactivity of cationic AuxAlyOz+ clusters,42,43 as well as the electronic and geometric structures of

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neutral Au2(AlO)244 have been reported in literature, while the reactivity of neutral AuxAlyOz

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clusters has not been touched yet.

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2. METHODS

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2.1. Experimental Methods

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The details of the experimental setup have been described in a previous study.41 Only a brief

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outline of the experiments is given below. The neutral AuxAlyOz clusters were generated by

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pulsed laser ablation of a rotating and translating Au/Al mixed disk (Au/Al molar ratio of 10:1)

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in the presence of 2% O2 seeded in a He carrier gas (99.999%) with a backing pressure of about

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5 atm. The clusters generated in a gas channel were expanded and reacted with pure reactant

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gases of CH4 or CD4 in a fast flow reactor in which the pressure of carrier gas was about 30 Pa.

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After the reactions, the charged clusters were removed from the molecular beams by two

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deflection plates. The neutral reactants and products were skimmed into the vacuum system of

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the TOF mass spectrometer and ionized by four vacuum ultraviolet (VUV) laser beams (118 nm,

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10.5 eV/photon) generated with an intense λ = 355 nm laser beam in a gas cell containing the


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Xe/Ar mixture. Most of the metal oxide clusters have ionization energies below 10.5 eV.45 After

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the single photon ionization with the 118 nm laser, the cluster ions were detected by a dual

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microchannel plate detector. The signals from the detector were recorded with a digital

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oscilloscope.

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2.2. Theoretical Methods

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Density functional theory (DFT) calculations using Gaussian 09 program46 were carried out to

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investigate the structures of Au2Al2O3 and Au2Al2O4 clusters, and the reaction mechanisms with

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methane. The TZVP basis sets47 for C, H, O and Al atoms and the D95V basis sets combined

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with the Stuttgart/Dresden relativistic effective core potentials (denoted as SDD in Gaussian

10

software) for Au were adopted. The TPSS functional48 has been proved to perform well for the

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Au−Al−O system,42 thus, the results by TPSS were given throughout this work. A Fortran code

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based on genetic algorithm was used to generate initial guess structures of the clusters. The

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reaction mechanism calculations involved geometry optimization of reaction intermediates (IMs)

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and transition states (TSs) through which the IMs transfer to each other. The TSs were optimized

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using the Berny algorithm method.49 The initial guess structures of the TSs species were

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obtained through relaxed potential energy surface scans using single and multiple internal

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coordinates. Vibrational frequency calculations were performed to check that the IMs or TSs

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have zero and only one imaginary frequency, respectively. Intrinsic reaction coordinates (IRC)

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were employed to check that a transition state connects two appropriate local minima. The zero-

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point vibration corrected energies (∆H0) were reported in this work. Natural bond orbital (NBO)

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analysis was performed with NBO 3.1.50

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The Rice−Ramsperger−Kassel−Marcus theory (RRKM) and RRKM-based variational

23

transition state theory (VTST)51 were used to calculate the rate constants of traversing transition


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states from intermediates and for CH4 desorption from adsorption complexes, respectively. For

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these calculations, the energy of the initially formed reaction intermediate (E) and the energy

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barrier (E≠) for each step were needed. The reaction intermediate possesses the vibrational

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energies (Evib) of Au2Al2O3 and CH4, the center of mass kinetic energy (Ek), and the binding

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energy (Eb) between Au2Al2O3 and CH4. The values of Evib and Eb were taken from the DFT

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calculations and Ek = µv2/2, in which µ is the reduced mass and v is the velocity (≈ 1000 m/s).

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The densities and the numbers of states required for RRKM and VTST calculations were

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obtained by the direct count method52 with the DFT-calculated vibrational frequencies under the

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approximation of harmonic vibrations.

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Figure 1. TOF mass spectra for the reactions of neutral AuxAlyOz clusters (a) with CH4 (b, c),

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and CD4 (d). The reactant gas pressures are shown and the reaction time is about 60 µs. AuxAlyOz

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is denoted as x, y, z and AuxAlyOzX is denoted as x, y, z, X (X = H, D, CH4, CD4).


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3. RESULTS

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3.1. Experimental Results

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The TOF mass spectra for the interactions of AuxAlyOz clusters with CH4 and CD4 are shown

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in Figure 1. Upon the interactions with CH4 in the fast flow reactor (Figures 1b and 1c), the

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signal magnitudes of Au2Al2O3 and Au2Al2O4 clusters decreased significantly, while those of

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other clusters did not change within the experimental uncertainties. Meanwhile, product peaks

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assigned as Au2Al2O3H+ and Au2Al2O4(CH4)1-2+ were obviously observed. The assignments of

8

the product peaks were further confirmed in the isotopic labeling experiments with CD4 (Figure

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1d). The adsorption product peak Au2Al2O3CD4+ could also be identified clearly while the

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corresponding Au2Al2O3CH4+ peak was overlapped with Au2Al2O4+ peak in Figure 1b, c. The

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pseudo-first-order rate constant (k1) for the depletion of Au2Al2O3 with respect to pressure

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increase of CH4 in the reaction is estimated to be (5 ± 3) × 10–12 cm3 molecule–1 s–1. According to

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the experiments, the interesting channel for production of Au2Al2O3H+ is found in the following

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reaction:

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Au2Al2O3 + CH4 + VUV → Au2Al2O3H+ + CH3• + e–

(1)

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Two possible mechanisms can be proposed for the above reaction: (i) the product of Au2Al2O3H

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was generated directly from the reaction of neutral Au2Al2O3 cluster with CH4 and then ionized

18

by VUV to generate Au2Al2O3H+, as shown below:

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Au2Al2O3 + CH4 → Au2Al2O3H + CH3•

(2a)

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Au2Al2O3H + VUV → Au2Al2O3H+ + e–

(2b)

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(ii) The Au2Al2O3H+ ion was produced from the ionic reaction following VUV ionization of

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methane adsorption intermediates, suggesting the following reactions:

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Au2Al2O3 + CH4 → Au2Al2O3CH4

(3a)


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Au2Al2O3CH4 + VUV → (Au2Al2O3CH4+)∗ + e–

(3b)

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(Au2Al2O3CH4+)∗ → Au2Al2O3H+ + CH3•

(3c)

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3.2. Theoretical Results

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The DFT calculations were carried out to get a mechanistic insight into the reaction details of

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methyl radical elimination observed in experiments. The lowest-energy isomer of Au2Al2O3

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(Figures 2 and S1) has a closed-shell electronic structure and the two Au atoms are separately

7

bonded with one aluminium atom and one terminally-bonded oxygen atom of the Al2O3 unit.53

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The natural charges of the two Au atoms are –0.20 e and +0.54 e, respectively. The structure of

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the corresponding cationic cluster Au2Al2O3+ shown in Figure 2 (right) is a doublet and the

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unpaired spin densities are mainly distributed around the two bridging oxygen atoms. The Au–

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O–Al bond angle of Au2Al2O3+ cluster increases obviously in comparison with the neutral

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cluster. It is clear that the electron detachment from Au2Al2O3 cluster induces significantly

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structural change.

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Figure 2. DFT-calculated structures of the neutral Au2Al2O3 cluster (left) and the corresponding

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cationic cluster (right). The natural charges (in e) and the Mulliken spin density values (in µB) are

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listed in parentheses and brackets, respectively. Bond lengths (in pm) are also shown.

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The DFT calculated enthalpy of the reaction between neutral Au2Al2O3 cluster and CH4

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(Equation 2a) is +0.39 eV, indicating that the direct production of Au2Al2O3H and a methyl

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radical from Au2Al2O3/CH4 couple is endothermic and not accessible under thermal collision


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Figure 3. DFT calculated potential-energy profile for VUV ionization induced reaction of

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neutral Au2Al2O3 cluster with CH4 (R). The relative energies (eV) of the reaction intermediates

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(I1–I5), transition states (TS1–TS4), and products (P1) are with respect to the separated reactants

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(R). The relative energies (eV) of the separated reactants (R+: Au2Al2O3+ and CH4), reaction

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intermediates (I6–I9), transition states (TS5–TS7), and products (P2) are with respect to I1+. The

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structures of I1–I9 are plotted and those of TS1–TS7, P1, and P2 can be found in the Supporting

8

Information. The vertical ionization energy of I1 and the bond lengths (in pm) are also shown.


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conditions. The reaction pathway for the reaction of Au2Al2O3 + CH4 is shown in Figure 3. The

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CH4 interacts with the positively charged Au atom in Au2Al2O3 cluster (Figure 2) to form the

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encounter complex (I1) with a binging energy of 0.66 eV. Then the reaction proceeds with the

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C–H bond activation by a transition state (TS1) with an overall positive barrier (0.03 eV). The

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theoretical collision rate of the encounter complex (I1) with the buffer gas (about 30 Pa He and

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30 Pa CH4 in our experiments) is about 2.7 × 107 s–1.54-56 The RRKM and VTST calculations51

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indicate that the rate of back dissociation (I1 → Au2Al2O3 + CH4) is 1.5 × 108 s–1, while the rate

8

of internal conversion (I1 → TS1 → I2) is 4.3 × 104 s–1. It means that a fair amount of the

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encounter complex (I1) can dissociate back to the separated reactants and the remaining part can

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be stabilized through collisions with the buffer gas. Even if the encounter complex (I1) has

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chances to overcome the barrier of 0.69 eV (I1 → TS1), the reaction complex can relax easily (I2

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→ I3 → I4 → I5) to form a more stable intermediate I5 of which the dissociation into separated

13

products Au2Al2O3H and CH3• (P1) is endothermic. As a result, the methyl radical generated

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directly from the reaction of neutral Au2Al2O3 cluster with CH4 is unfavorable both

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thermodynamically and kinetically.

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In sharp contrast to the neutral Au2Al2O3 cluster, the ionic Au2Al2O3+ cluster is reactive

17

toward CH4.43 The product ions Au2Al2O3H+, Au2Al2O3CH4+ (Figure 1), and AuAl2O3H2+

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(Figure S3) detected by VUV ionization in this experiment are all observed in the reaction

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between Au2Al2O3+ cluster and CH4 in an ion trap reactor. Therefore, we propose that VUV

20

ionization of the neutral adsorption intermediate creates reactive cation Au2Al2O3CH4+ to induce

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C–H bond activation and the methyl radical formation, corresponding to reaction (3). The

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possible reaction mechanism is also shown in Figure 3. The association product I1 is vertically

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ionized by VUV on a very short time scale to produce reactive cation I1+. Most of the photon


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energy above 8.61 eV is removed by the exiting photoelectron and a small fraction of it can be

2

deposited in the cluster.57,58 The cation I1+ subsequently undergoes configuration changes of

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Au2Al2O3 moiety as shown in Figure 2, and forms the stable intermediate I6 (–0.42 eV). Then

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the H atom transfers from C atom to a bridging O atom (I6 → TS5 → I7) and this step needs

5

energy that can be from the excess photon energy and/or the binding energy between Au2Al2O3

6

and CH4 to overcome the overall positive barrier of 0.35 eV. A portion of I6 cannot overcome

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the barrier (TS5), which interprets the experimental observation of the association complex

8

(Au2Al2O3CD4+) in Figure 1. Note that the cationic complex I6 cannot dissociate back to the

9

separated Au2Al2O3+ (right in Figure 2) and CH4 because of the high binding energy (1.06 eV)

10

between the two species. To proceed to the remaining part of the reaction, the H atom in I7 needs

11

to transfer further to terminally bonded O atom and AuCH3 moiety moves concomitantly to

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another bridging O atom (I7 → TS6 → I8 → TS7 → I9), the process of which releases enough

13

energy to evaporate CH3•. The production of CH3• from the VUV ionization of neutral

14

intermediate I5 (Figure S3) is also feasible by theoretical calculations although the formation of

15

I5 (rate: 4.3 × 104 s–1) is much less favorable than the stabilization of I1 through bath gas cooling

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(rate: 2.7 × 107 s–1).

17

4. DISCUSSIONS

18

A methyl radical was generated from the cationic species created by VUV ionization of

19

neutral association complex Au2Al2O3CH4, while only the association product was observed for

20

Au2Al2O4 cluster at the same experimental conditions. Note that the structural characterization of

21

the association product is technically possible by collisional induced dissociation or infrared

22

photodissociation. However, it is difficult to perform the experiments for Au2Al2O4CH4+ because

23

the VUV generated ion intensity in this study is very low (~ 1 ion/10 laser pulse). To understand


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the difference of the two reaction systems, the structures of Au2Al2O4 cluster (Figure S4) and the

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methyl radical elimination mechanisms (Figure S5) were calculated. Similar to the

3

Au2Al2O3/CH4 couple, the reaction between the neutral Au2Al2O4 cluster and CH4 is also unable

4

to produce CH3•. A similar structural relaxation occurs upon the newly formed cation

5

(Au2Al2O4CH4+)*, while the relaxation energy related to the geometry changes is significantly

6

smaller in Au2Al2O4CH4+ (0.21 eV) than that in Au2Al2O3CH4+ (0.42 eV). There is not enough

7

energy to surmount the barrier in the VUV ionization induced Au2Al2O4CH4+ reaction system.

8

The reaction of Au2Al2O4+ cluster with CH4 in the ion trap reactor has also been experimentally

9

studied and compared with Au2Al2O3+ reaction system (Figure S6). It was found that the

10

branching ratio of association channel (16%) for generation of Au2Al2O4CH4+ is larger than that

11

(2%) of Au2Al2O3CH4+, which is qualitatively consistent with the experimental observation in

12

the VUV ionization induced reactions of Au2Al2O3 and Au2Al2O4 clusters with CH4. Thus, the

13

experimental and theoretical results demonstrate the reactivity difference of the two reaction

14

systems and further verify the selectivity of VUV ionization induced reactions.

15

The VUV single photon ionization coupled with TOF-MS has been demonstrated to be

16

essential for detecting neutral species.59 The reactivity of neutral oxide clusters of

17

manganese,60,61 cobalt,62 iron,57,63 and vanadium58,64,65 toward CO, C2H4, NH3, C4H10, and

18

CH3OH has been studied in recent years. It was found that the products detected by VUV

19

photoionization were generated directly from the reactions of neutral metal oxide clusters with

20

small molecules. In sharp contrast, the reaction of neutral Au2Al2O3 cluster with methane studied

21

in this work is incapable of producing the methyl radical. Therefore, the reaction mechanism that

22

VUV ionization generated reactive ion Au2Al2O3CH4+ can further transform methane has been

23

proposed. It is noteworthy that similar ion-molecule reactions occurring upon newly formed


12

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1

ionized clusters have been identified in many molecular cluster systems such as (H2O)n, (NH3)n,

2

(CH3OH)n and so on.66-70

3

Photocatalytic conversion of methane has been widely investigated in condensed-phase

4

systems.71-73 Two different mechanisms74 have been proposed for methane activation. The active

5

sites of the catalysts are excited by photon energy and then methane is adsorbed and activated.

6

For example, the CH3• can be produced in photocatalytic oxidation of CH4 over oxide catalysts

7

through two steps:75 O2– + hν → O– + e– and O– + CH4 → OH– + CH3•. Alternatively, the

8

reactive sites are activated by the photon energy after methane adsorption. For example, the CH3•

9

can be generated with MxOyCH4 + hν → MxOyCH4+ + e– and MxOyCH4+ → MOH+ + CH3•. In

10

this study, the methane adsorption on the neutral cluster (Au2Al2O3) and the excitation of the

11

adsorption complex by VUV ionization to convert methane to CH3• provide a molecular

12

evidence for the second mechanism.

13

5. CONCLUSION

14

In summary, the reactions of neutral heteronuclear metal oxide clusters AuxAlyOz (x =1, 2)

15

with methane have been investigated for the first time by employing time-of-flight mass

16

spectrometer coupled with a vacuum ultraviolet laser system. The methyl radical elimination and

17

molecular adsorption have been experimentally observed for the interactions of Au2Al2O3 and

18

Au2Al2O4 with CH4, respectively. Theoretical calculations demonstrated that the methyl radical

19

generated directly from the reaction of neutral closed-shell Au2Al2O3 cluster with CH4 is

20

unfavorable both thermodynamically and kinetically. We propose that vacuum ultraviolet

21

ionization of the neutral association product (Au2Al2O3CH4) creates reactive cluster cation

22

(Au2Al2O3CH4+)∗, which undergoes very fast intracluster conversion to generate a methyl

23

radical. This photoionization induced methane conversion offers molecular-level insights into the


13


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methane activation and conversion mediated with photocatalytic materials.

2


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ASSOCIATED CONTENT

2

Supporting Information

3

The Supporting Information is available free of charge on the ACS Publications website. Details

4

regarding experimental and theoretical methods, additional TOF mass spectra, and DFT

5

calculated structures and reaction mechanisms.

6

AUTHOR INFORMATION

7

Corresponding Author

8

*E-mail: [email protected]; [email protected];

9

Phone: +86-10-62568330; Fax: +86-10-62559373.

10

ORCID

11

Yan-Xia Zhao: 0000-0002-4425-5211

12

Sheng-Gui He: 0000-0002-9919-6909

13

Notes

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The authors declare no competing financial interests.

15

ACKNOWLEDGMENT

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This work was financially supported by the Chinese Academy of Sciences (No. XDA09030101),

17

and the National Natural Science Foundation of China (Nos. 21773253, 91645203, and

18

21573247).

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Gascon, J. Strategies for the Direct Catalytic Valorization of Methane Using Heterogeneous

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Catalysis: Challenges and Opportunities. ACS Catal. 2016, 6, 2965-2981.

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1


FIGURES

2 3

Figure 1. TOF mass spectra for the reactions of neutral AuxAlyOz clusters (a) with CH4 (b, c),

4

and CD4 (d). The reactant gas pressures are shown and the reaction time is about 60 µs. AuxAlyOz

5

is denoted as x, y, z and AuxAlyOzX is denoted as x, y, z, X (X = H, D, CH4, CD4).

6


23


Page 24 of 26

1 2

Figure 2. DFT-calculated structures of the neutral Au2Al2O3 cluster (left) and the corresponding

3

cationic cluster (right). The natural charges (in e) and the Mulliken spin density values (in µB) are

4

listed in parentheses and brackets, respectively. Bond lengths (in pm) are also shown.

5 6


24

Page 25 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1 2

Figure 3. DFT calculated potential-energy profile for VUV ionization induced reaction of

3

neutral Au2Al2O3 cluster with CH4 (R). The relative energies (eV) of the reaction intermediates

4

(I1–I5), transition states (TS1–TS4), and products (P1) are with respect to the separated reactants

5

(R). The relative energies (eV) of the separated reactants (R+: Au2Al2O3+ and CH4), reaction

6

intermediates (I6–I9), transition states (TS5–TS7), and products (P2) are with respect to I1+. The

7

structures of I1–I9 are plotted and those of TS1–TS7, P1, and P2 can be found in the Supporting

8

Information. The vertical ionization energy of I1 and the bond lengths (in pm) are also shown.


25


1

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TOC

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VUV Ionization Induced Reaction of Neutral Au2Al2O3 Clusters With

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